1. Field of the Invention
The invention relates to fluorescent lamps, and is directed more particularly to a base for such lamps.
2. Description of the Prior Art
In the manufacture of lamps, a lamp envelope is usually provided with at least one base. Generally, a lamp base comprises a rigid shell secured to an end portion of the lamp envelope. At least one insulating disk is fixed in the shell for carrying one or more hollow base pins, or contacts, into which the lamp lead wires are electrically secured. The lamp is supported by one or more holders, or sockets, into which the lamp bases extend for communication with a source of electrical energy.
Typically, such lamp bases are secured to the end portions of the lamp envelope by means of a cement which is applied to the inside surface of a base shell annular wall. A sufficient quantity of cement is used to fill a gap between a lamp seal and the annular wall of the base. During manufacturing, each base is first fitted loosely onto a respective end portion of the lamp envelope. Thereafter, the cement is cured, as by heating, which allows the base to adhere to the lamp bulb and withstand industry torque requirements.
While the above technique of securing the lamp base by means of a suitable cement has been employed successfully in many lamp families, including fluorescent, it has been found that certain disadvantages exist. For example, the cement not only adds cost to the lamp but also requires the need for a separate process of applying the raw cement to the base shell. Moreover, while present manufacturing facilities using such a technique are equipped with machines which dispense cement, the machines require constant monitoring and periodic mechanical and electrical maintenance. Another disadvantage is the curing process of the cement, wherein indirect natural gas flame heat is used to cure the basing cement after the base is fitted to the end of the lamp. The temperatures required to cure the cement sometimes cause damage in the seal area of the lamp envelope. In addition, the machinery needed to provide the heat for curing not only requires periodic maintenance but also takes up valuable floor space in the production line.
Various alternatives for securing the base to the lamp end with little or no cement, or other type of adhesive, have been proposed in the past. For example, U.S. Pat. No. 2,993,191, which issued on Jul. 18, 1961 to Pietzsch et al, discloses a base for an electric discharge lamp wherein the base is constructed from resin having a modulus of elasticity which is greater than 5,000 kg./cm.2 and as high as about 19,000 kg./cm.2. The resin has a breaking dilation of more than 50% and as high as about 230% and has an initial softening temperature of as low as about 150° Celsius and as high as about 210° Celsius. In one embodiment, the base of Pietzsch et al is positioned with the annular wall adjacent to the trough of a bulb end which has been heated to about 330° to 350° Celsius. As a result, the base material melts and occupies the trough or channel and, by reason of the character of the material of which the base is composed, adheres to the surface of the glass bulb. Alternatively, the base may be pressed against the bulb end to cause an annular rib or reinforcement to snap over the bead or rim and into a trough or channel of the bulb without heating the bulb neck.
U.S. Pat. No. 4,221,453, which issued to Wagener on Sep. 9, 1980, discloses a base for a fluorescent lamp. The base comprises a frontal portion, contact pins electrically connected to the connecting wires, at least one drop of glue which dries at room temperature, and an annular wall extending circumferentially from and perpendicular to the frontal portion. The annular wall has at least two equally circumferentially spaced knobs protruding inwardly. The base is formed from an elastic, bendable thermoplastic material so that when the base is fitted to the tube over the end portion, the annular wall elastically deforms and the knobs slide over the collar and snap into the groove of the lamp end portion. To safeguard against rotary movement of the base relative to the bulb, the base is formed with radial ribs to be disposed in notches provided in the bulb neck.
U.S. Pat. No. 5,432,400, which issued Jul. 11, 1995 to Spaulding et al, discloses a lamp including a glass envelope having a base fitted at each end portion thereof. Each lamp base includes a metallic base shell having an annular flange. The annular flange is heated prior to fitting over the end portion of the envelope so as to increase the inner flange diameter. Cooling of the annular flange after fitting reduces the flange diameter, thereby providing an interference fit with the end portion. The lamp base is retained on the end portion without the need for basing cement. In order to accommodate variations in the diameter of the lamp seals, an annular rib is formed on the inner surface of the flange. After cooling, the annular rib forms an interference fit with the lamp end portion.
While the bases described in the above patents appear to be satisfactory from a functional standpoint, it is believed that unanticipated production and/or other related problems, as well as material cost, may explain why such bases have not been commercially successful. Accordingly, it is deemed advantageous to provide another viable alternative.